Wireless communication networks are known. In a cellular system, radio coverage is provided to user equipment, for example, mobile telephones, by geographical area. Those geographical areas of radio coverage are known as cells. A base station is located in each geographical area to provide the required radio coverage. A base station may support more than one cell providing coverage in the same geographical area. User equipment in the area served by a base station receives information and data from a base station and transmits information and data to a base station.
Information and data transmitted by a base station to user equipment occurs on channels of radio carriers known as downlink carriers. Information and data transmitted by user equipment to a base station occurs on uplink channels of radio carriers known as uplink carriers.
In wireless telecommunications networks, user equipment can move between geographical base station coverage areas. Services provided to user equipment are typically overseen by a radio network controller (RNC). The RNC communicates with user equipment and base stations and determines which base station each user equipment may be primarily connected to. Furthermore, the RNC acts to control and communicate with a base station and user equipment when user equipment moves from a geographical area served by one base station to a geographical area served by another base station, or between geographical areas served by the same base station.
In some cellular systems, for example, in a UMTS system, functionality known as “soft handover” (SHO) can be implemented. That functionality allows data traffic transmitted by user equipment or other mobile terminals using uplink channels to be received and decoded by multiple base stations. Those base stations may also be known in a UMTS system as “node Bs”. Furthermore, if utilizing soft handover techniques, a user equipment may receive and combine data and control signals from multiple base stations in the downlink. That is to say, the user equipment can receive or listen to transmissions from multiple base stations and multiple base stations may listen to transmissions made by user equipment. This has several advantages including that transmissions made by user equipment are not seen as interference by adjacent cells. In order that such soft handover techniques can be implemented, it is necessary for user equipment and the relevant nodeBs to known that they are allowed and expecting to communicate with one another. User equipment is typically informed by the network of those nodeBs by which its transmissions can be heard and to which it may listen. That list of base stations is referred to as the “active set” of that user equipment.
Typically, if implementing a soft handover regime, a radio link is first set up between user equipment and a single base station. If it is determined that entry to a soft handover regime is desirable and possible, necessary information about the configuration of existing uplink transmissions from the user equipment to the single base station, including for example, scrambling code being used, can be passed from a first base station via a radio network controller (RNC) to a second base station to be added to the active set of the user equipment. That information passed to a second base station enables the second base station to synchronize its operation to the user equipment's uplink transmissions. Once a second node B has obtained uplink synchronization, the second base station can set up a new downlink with the user equipment such that its transmission timing is likely to be successfully received at the user equipment close to the reception timing of the existing downlink between the first base station and the user equipment.
Use of soft handover techniques can lead to improved network performance, since by allowing more than one base station to communicate with user equipment it is possible to obtain a degree of selective combination through macro diversity and soft combinations leading to gain. Soft handover functionality also allows an opportunity to minimise interference caused between cells by user equipment operating at the edge of geographical coverage regions supported by adjacent base stations.
As the configuration and arrangement of network nodes such as base stations within a wireless communication network becomes increasingly complex, and functionality of user equipment is increased, unexpected situations can occur which can lead to decreased overall network performance.
Accordingly, it is desired to provide an improved technique for facilitating use of soft handover techniques to reduce interference within a network.